Cryptosporidium is a water-borne parasitic protozoan responsible for the water-borne disease Cryptosporidiosis. Outbreaks of Cryptosporidiosis have been attributed to ingestion of drinking water, recreational water or food containing viable oocysts of Cryptosporidium. Cryposporidium oocysts are typically introduced into the water through contamination of the water with fecal matter from cattle or humans containing oocysts. The oocysts have a hard outer cell wall that renders the oocysts resistant to the effects of chlorine present at concentrations typical of drinking water and recreational water. The oocyts are approximately 4–6 microns in size, which makes them difficult to remove by filtration. Since filtration and chlorination are universally practiced as a means for clarifying and sanitizing drinking water in municipal water treatment facilities and for maintaining the clarity of recreational water such as in swimming pools, water parks, hot tubs, baths and spas, the chlorine resistance and size of the oocysts make it difficult to ensure that water is free of this disease-causing microorganism.
A variety of filters and filter medias are used to clarify water in swimming pools, water parks, hot tubs and spas. Sand filters are common for swimming pool use and municipal water treatment. Diatomaceous earth filters are also available for use in swimming pools and water parks. Cartridge filters available to both pools and spas utilize a synthetic fabric enclosed in a plastic cartridge. Different filter media exhibit different capabilities for removing particles that vary in size. Sand filters are capable of filtering out particles in the size range of 20–25 microns, while cartridge filters are typically capable of removing particles in the size range of 5–10 microns. Diatomaceous earth filters exhibit the capability of removing particles in the size range of 1–3 microns, but have to be replaced frequently.
Coagulation and flocculation followed by filtration is commonly utilized in the treatment of drinking and recreational water to remove suspended microscopic particles. Non-filterable suspended microscopic particles tend to possess an electrostatic charge that prevents the particles from aggregating into larger filterable aggregates due to charge-charge repulsion. This can be often overcome through the use of coagulants and flocculants. Coagulants are chemicals, that when dissolved in water, form ions of charge opposite to that of the suspended particles. The charge interaction of the coagulant with the particles results in the reduction of the particle's charge or so called zeta potential. Reduction of the particle's zeta potential reduces particles' charge-charge repulsion and allows the particles to come sufficiently close together to form aggregates large enough to be filtered out. The most commonly used coagulants are metal salts such as aluminum sulfate and ferric chloride and their use is highly dependent on both pH and dosage.
Flocculants are typically water soluble or water dispersible high molecular weight polyelectrolyte long chain polymers composed of repeating monomeric units that can be categorized into inorganic or organic compounds. The inorganic polyelectrolytes are polymerized metal salts and may include polyaluminum hydroxychloride, polyaluminum silicate sulfate and polyaluminum sulfate. Organic polyelectrolyte flocculants are derived synthetically or obtained from natural sources. The organic polyelectrolytes can exist as charged or uncharged polymers depending on their composition. Flocculants when added to water containing aggregates of microscopic particles or non-aggregated particles exhibit the ability to bind and gather the particles or particle aggregates into even larger aggregates that can be easily filtered. The success of this aggregation is dependent on a variety of properties unique to the particles or particle aggregates and the properties of the particular flocculant being used. The stability of the flocculated particles or aggregated particles can be important to successful removal by filtration. Unstable flocculated particles or particle aggregates may come apart during filtration and pass through the filter while only the more stable aggregates are retained. Aggregate stability can be influenced by the flow rate and pressure across the filter and the turbulence of the water.
Previous attempts at removing Cryptosporidium oocysts via filtration from large bodies of water moving at high flow rates have not been successful. Since Cryptosporidium oocysts are negatively charged, coagulants and flocculants such as ferric sulfate, ferric chloride, aluminum sulfate or polyaluminum chloride have been tried unsuccessfully as a means to remove the oocysts from water through the process of aggregation, settling and filtration. Although flocs of oocysts are formed using these particular coagulants, the Cryptosporidium oocyst flocs are unstable and subject to hydrodynamic shear forces that make them susceptible to breaking up and coming apart resulting in their not being retained on filters. The use of anionic or cationic polymeric polyelectrolyte flocculants has been suggested as a means to stabilize Cryptosporidium oocyst-containing flocs against shear. One such study involving dissolved air flotation (DAF) was performed using ferric sulfate as the primary coagulant and LT22, a cationic acrylamide co-polymer. No improvement in oocyst removal over that expected with ferric sulfate coagulant alone was observed. Sand filtration for removing Cryptosporidium oocysts from water has been evaluated with some success. Rapid sand filtration has been reported to remove 3 logs of Cryptosporidium oocysts. Slow sand filtration that utilizes a finer grain of sand and a slower flow rate was reported in a pilot scale study to be fairly good in removing Cryptosporidium oocysts without having to use flocculants or coagulants. The previous coagulants and flocculants cannot be used in conjunction with slow sand filtration because they tend to clog the pores and severely restrict the flow rate.
Currently there is no methodology that is effective in stabilizing flocs of Cryptosporidium oocysts for their significant removal from water through aggregation followed by filtration. Accordingly, there is a need to find a flocculant or coagulant or a combination of the two for obtaining significant removal of oocysts that can take advantage of existing filtration technology such as sand, diatomaceous earth or synthetic cartridge filters to provide safe water for recreation and drinking.